1. Field of the Invention
This invention relates to the 37-kDa Streptococcus pneumoniae surface adhesin A protein. Specifically, the invention relates to an isolated nucleic acid encoding the 37-kDa protein of Streptococcus pneumoniae, to unique fragments of the nucleic acid encoding the 37-kDa protein of Streptococcus pneumoniae, and to the polypeptides encoded by those nucleic acids. The invention further relates to antibodies to those polypeptides, and to methods of detecting the presence of Streptococcus pneumoniae, methods of preventing Streptococcus pneumoniae infection, and methods of treating a Streptococcus pneumoniae infection.
2. Background Art
Pneumococcal disease continues to be a leading cause of sickness and death in the United States and throughout the world. Both the lack of efficacy of the currently used polysaccharide vaccines in children under 2 years of age and their variable serotype-specific efficacy among vaccinated individuals, have prompted manufacturers to investigate alternative vaccine formulations that do not require the use of multiple capsular polysaccharides. One current approach under consideration is the use of immunogenic species-common proteins as vaccine candidates. These proteins could be used in combination with other immunogenic proteins or as protein carriers in a protein-polysaccharide or oligosaccharide conjugate vaccine. An effective vaccine that included a common protein could eliminate the need for formulations based on multiple capsular polysaccharides (as in the 23-valent polysaccharide vaccine) by offering a broader range of protection against a greater number of serotypes. Additionally, a protein-based vaccine would be T-cell dependent and provide a memory response, resulting in a more efficacious vaccine.
Of the reported pneumococcal proteins, only pneumolysin and the pneumococcal surface protein A (PspA) have been extensively examined for their suitability as vaccine candidates. While both have been shown to be partially protective in mice (Paton et al. 1983. "Effect of immunization with pneumolysin on survival time of mice challenged with Streptococcus pneumoniae." Infect. Immun. 40:548-552 and McDaniel et al. 1991. "PspA, a surface protein of Streptococcus Pneumoniae, is capable of eliciting protection against pneumococci of more than one capsular type." Infect. Immun. 59:222-228), there are disadvantages to their use as vaccine immunogens. Pneumolysin, although well conserved among pneumococci, has been shown to have strong toxic effects in its native state (AlonsoDeVelasco et al. 1995. "Streptococcus pneumoniae: Virulence factors, pathogenesis, and vaccines." Microbiol. Rev. 59:591-603). Recombinant derivatives of reduced toxicity have been produced, and while they show promise in animal protection studies (Alexander et al. 1994. "Immunization of mice with pneumolysin toxoid confers a significant degree of protection against at least nine serotypes of Streptococcus pneumoniae. Infect. Immun. 62:5683-5688) the problem of maintaining maximal immunogenicity and eliminating toxicity to humans is still in question. PspA, on the other hand, is serologically and structurally heterogeneous. (Crain et al. 1990. "Pneumococcal surface protein A (PspA) is serologically highly variable and is expressed by all clinically important capsular serotypes of Streptococcus pneumoniae." Infect. Immun. 58:3293-3299). Its use in vaccine formulations would require multiple PspA types, thus increasing the complexity of vaccine preparation.
An immunogenic species-common protein has been identified from Streptococcus pneumoniae. (Russell et al. 1990. "Monoclonal antibody recognizing a species-specific protein from Streptococcus pneumoniae." J. Clin. Microbiol. 28:2191-2195 and U.S. Pat. No. 5,422,427 in which the 37-kDa protein is referred to as pneumococcal fimbrial protein A). The 37-kDa S. pneumoniae protein has been the focus of several studies and has been designated pneumococcal surface adhesin protein A (PsaA). Immunoblot analysis studies using anti-PsaA monoclonal antibody showed that PsaA is common to all 23 pneumococcal vaccine serotypes (Russell et al. 1990). Enzyme-linked-immunosorbent assay studies have indicated that patients with pneumococcal disease show an antibody increase in convalescent-phase serum to PsaA compared with acute-phase serum antibody levels (Tharpe et al. 1995. "Purification and seroreactivity of pneumococcal surface adhesin A (PsaA)." Clin. Diagn. Lab. Immunol. 3:227-229 and Tharpe et al. 1994. "The utility of a recombinant protein in an enzyme immunoassay for antibodies against Streptococcus pneumoniae." abstr. V-2, p. 617. 1994. American Society for Microbiology, Washington, D.C.). Additionally, a limited in vivo protection study showed that antibodies to the 37-kDa protein protect mice from lethal challenge (Talkington et al 1996. "Protection of mice against fatal pneumococcal challenge by immunization with pneumococcal surface adhesin A (PsaA)." Microbial Pathogenesis 21:17-22).
The gene encoding PsaA from S. pneumoniae strain R36A (an unencapsulated strain) has been cloned in Escherichia coli and sequenced, but this serotype does not contain a 37 kDa protein encoding nucleic acid that is highly conserved among the various serotypes. (Sampson et al. 1994. "Cloning and nucleotide sequence analysis of psaA, the Streptococcus pneumoniae gene encoding a 37-kilodalton protein homologous to previously reported Streptococcus sp. adhesins." Infect. Immun. 62:319-324). This particular nucleic acid and polypeptide, therefore, are of limited value for use as diagnostic reagents, in infection prevention, in infection treatment, or in vaccine development.
Sequence conservation is a necessary requirement for a candidate species-common vaccine. At present, there are no studies that have investigated the sequence conservation of the psaA gene among pneumococcal types, specifically among encapsulated pneumococci which cause the vast majority of serious disease. Therefore, a need exists to investigate the conservation of the gene in order to provide a polypeptide which can serve as a vaccine for multiple strains of Streptococcus pneumoniae. The present invention fulfills that need by analyzing psaA genes from the 23 serotypes in the 23-valent polysaccharide vaccine and by providing a polypeptide and antibodies to that polypeptide which are conserved among the S. pnuemoniae serotypes and which confer protection to Streptococcus pneumoniae infection.